1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to golf balls comprising a core and a cover, with dimples being formed on the cover.
2. Description of the Related Art
General golf balls other than those manufactured for use in practice ranges comprise a core and a cover. There exist cores composed of a single solid rubber layer, two or more solid rubber layers, a synthetic resin layer in addition to a solid rubber layer, and the like.
Although there are a variety of characteristics required for a golf ball to golfers, senior golfers tend to place great importance on spin performances in particular. High back spin speed results in small run (a distance from the position where the golf ball dropped to a position where it stopped, also referred to as “roll”). In other words, for golfers, golf balls which are liable to be spun around are apt to be rendered to stop at a targeted position. High side spin speed results in possibility of curving of the golf ball. In other words, for golfers, golf balls which are liable to be side spun are apt to be rendered to intentionally curve. Golf balls that are excellent in spin performances are excellent in control performances. Senior golfers particularly place great importance on control performances upon impact with a short iron.
Golf balls having longer contact time period and greater contact area with a golf club face, upon impact with a golf club, are excellent in spin performances. Golf balls for use by senior golfers are formed with a cover made of a soft material, thereby aiming to increase the contact time period and contact area.
In regard to a thickness of a cover, a variety of investigations have been conventionally conducted. Golf balls formed with a cover made of a soft material tend to have improved spin performance and durability as the cover is thicker, and to the contrary, as the cover is thinner, they tend to have improved resilience performance. In other words, a golf ball comprising a soft and thick cover has a defect of inferior resilience performance, and a golf ball comprising a thin cover has a defect of inferior spin performance and durability even though the cover is soft. The spin performance and durability are reciprocal performances to the resilience performance.
A golf ball has from about 200 to 550 dimples on its surface. A role of the dimples involves causing turbulent flow detachment through promoting turbulent flow transition of a boundary layer by disrupting the air flow around the golf ball during flight. By promoting the turbulent flow transition, a detachment point of air from the golf ball shifts backwards leading to the reduction of a drag coefficient (Cd) so that flight distance of the golf ball is prolonged. In addition, a difference of detachment points on the upper and lower sides of the golf ball resulting from back spin is increased by the promotion of turbulent flow transition. Therefore, the lift force that acts on the golf ball is elevated.
Specifications of the cover exert an influence upon behavior of a golf ball at impact of the golf ball and a golf club. To the contrary, specifications of the dimples exert an influence upon aerodynamic characteristics rather than the impact. Specifications of the cover and specifications of the dimples are comprehended by the skilled person in this art as discrete factors in terms of performances of a golf ball.
In Japanese Patent Publication Reference JP-A-305114/1998, JP-A-57067/1999, JP-A-70414/2000 and JP-A-225209/2000, there are disclosed golf balls having combination of an optimized cover and optimized dimples.
Even with the techniques disclosed in these publications, specifications of the dimples have been merely comprehended as a factor that exerts an influence upon aerodynamic characteristics. Specifications of the dimples have not been necessarily comprehended as a factor that exerts an influence upon behavior of a golf ball at impact. With respect to the behavior of a golf ball at impact, there remains room for the improvement. Golfers have desired golf balls that are excellent in all terms of a spin performance, durability and resilience performance.